The present invention relates to a method of fabricating a suspended microstructure with a sloped support, and to a suspended microstructure fabricated by the method. More specifically, the present invention is concerned with manufacturing of three-dimensional suspended microstructures equipped with support members. These microstructures include: microbeams, microplatforms, and more complex structures consisting of multiple microbeams and microplatforms. The mentioned three-dimensional suspended microstructures are part of such miniature devices as: sensors of radiation, temperature, pressure, flow, chemical and biological species, emitters of radiation and others. These miniature devices are often classified as Micro Electro Mechanical Systems (MEMS), Micro Opto Electro Mechanical Systems (MOEMS) or simply Micro Systems (MS). The microstructures listed above may also be part of various electronic Integrated Circuits (ICs).
In the standard binary photolithographic pattern generation process applied to manufacturing of electronic integrated circuits and other microdevices, a series of masking steps, exposure steps and etching steps are used. In this process, a photoresist is applied on top of a substrate and a series of binary masks consisting of transparent and opaque regions are used in sequence to produce the final pattern. The process involves applying a binary mask, exposing the photoresist through the mask, developing the binary pattern transferred from the binary mask to the photoresist, and then dry or wet etching of the substrate using the photoresist as a masking layer. This sequence of operations is repeated with a second binary mask. In order to generate more complex patterns, it is usually necessary to repeat this masking, exposing, developing, etching sequence several times making use of several binary masks and maintaining the required registration of the masks during the successive manufacturing sequences. An example of a binary mask photolithographic pattern transfer process is schematically shown in FIGS. 1A to 1F.
A different way of generating three-dimensional micropatterns consists of utilizing a so-called grey scale mask. A grey scale mask is a two-dimensional surface with varying optical transmittance. The variation of the optical transmittance represents three-dimensional information e.g., a height profile or depth pattern. The grey scale mask is used to transfer this three-dimensional information to a photoresist film deposited on a substrate by photoexposure and development, which leaves a modulated photoresist film thickness. The three-dimensional information now contained in the thickness modulated photoresist film may be subsequently transferred into the substrate by a known etching process, thereby creating the desired depth pattern in this substrate. The resulting processed substrate then contains, as a physical contour, the three-dimensional information that was originally represented by the variation of the optical transmittance of the grey scale mask. An example of a micropattern generation using a grey scale photomask is shown schematically in FIGS. 2A to 2C.
Several methods for the fabrication of grey scale masks are known in the art. U.S. Pat. No. 5,078,771 by Wu describes a method of making grey scale masks in a high energy beam sensitive glass article (HEBS glass article herein) comprising a body portion and an integral ion-exchanged surface layer containing a high concentration of silver ions. This surface layer becomes darkened upon exposure to high-energy beams without resorting to heat or other development steps. The high-energy beams used to expose the HEBS glass article include electron beams, various ion beams and laser beams. The HEBS glass articles are colorless and totally are transparent to actinic radiation before exposure to high-energy beams and not darkened by actinic radiation at intensities within, above and/or below those commonly used in photolithography. Actinic radiation is defined herein as radiation in the wavelength range of ultraviolet and/or longer wavelengths. The image recorded in the HEBS glass article with a high energy beam as well as the unexposed transparent area of the glass are stable indefinitely in all possible thermal, lighting and humidity conditions.
U.S. Pat. No. 5,310,623 by Gal describes a grey scale mask constructed with a plurality of precisely located and sized light transmitting openings. The openings are formed with sufficiently small specific opening sizes and are located at a sufficiently large number of specific locations, which locations are correlated to related locations on the configuration of the designed grey scale image.
U.S. Pat. No. 5,334,467 by Cronin et al. describes a two-level grey scale mask. One level is constructed of a glass made partially transmissive by substitution of silver ions in place of metal ions of alkali metal silicates employed in the construction of the glass. The second level is made opaque by construction of the layer of a metal such as chromium.
U.S. Pat. No. 5,998,066 by Block et al. describes a method of producing a high-resolution grey scale mask using an inorganic chalcogenide glass, such as selenium germanium, coated with a thin layer of silver.
Applications of grey scale mask technology to manufacturing of various micro optical components have been proposed. U.S. Pat. No. 5,482,800 by Gal, U.S. Pat. No. 6,033,766 by Block et al. and U.S. Pat. No. 6,107,000 by Lee et al. describe fabrication of miniature diffractive optical components such as diffractive microlenses and gratings, and computer generated holograms by photolithography and etching making use of grey scale masks.
An application of the grey scale photomask technology to fabrication of optical refractive micro components is proposed in U.S. Pat. No. 6,071,652 by Feldman et al., while the application of the grey scale masks to manufacturing of optical guided-wave devices is proposed in U.S. Pat. No. 5,480,764 by Gal et al.
However, none of the above patents proposes a method of fabricating a suspended microstructure with a support having mechanical and electromechanical properties that can be precisely controlled.
It is an object of the present invention to provide a method of fabricating a suspended microstructure with a sloped support, and a suspended microstructure fabricated by said method where the mechanical and electromechanical properties of the support can be precisely controlled.
According to the present invention, there is provided a method of fabricating a suspended microstructure with a sloped support, comprising the steps of:
(a) providing a member having three stacked up layers including a first substrate layer, a second temporary layer and a third photoresist layer;
(b) photolithographically transferring a sloped pattern to the third photoresist layer by means of a grey scale mask;
(c) etching the second layer through the third layer resulting from step (b) to obtain a surface with at least one continuous slope with a predetermined angle with respect to the first surface layer;
(d) depositing a fourth layer on the previous layers;
(e) etching the fourth layer to obtain the sloped support; and
(f) removing the second layer to obtain the microstructure with the sloped support.
According to a preferred embodiment, the method further comprises after step (e) and before step (f), steps of:
(i) depositing a fifth planarization layer for covering the previous layers except for a top portion of the sloped support;
(ii) depositing a sixth layer on the previous layers; and
(iii) etching the sixth layer to obtain a microplatform;
wherein step (f) further includes a removal of the fifth layer.